The potential hazards of operating vehicles such as tractors on steeply sloping surfaces has long been appreciated by those skilled in the art. Tractors utilized in farming operations or in highway right-of-way mowing, to name but two examples, frequently must travel along hillsides or other sloped paths where the tractor assumes an angle of attack with respect to the hill. This may take place either because the tractor is being driven more-or-less directly up the hill, or because the tractor is moving in a crabwise fashion across the hill to maintain a particular path of travel notwithstanding slippage down the slope. In either case, the front end of the tractor is elevated with respect to the back axle of the tractor, presenting an angle of attack with respect to horizontal. If this angle of attack exceeds some angle where the tractor's center of gravity moves behind the back axle, the tractor will overturn. Many serious and often-fatal accidents have occurred where tractor operators, whether through negligence or simple inattention, exceeded the maximum possible angle of attack for a particular tractor. The risk of overturn or tipping may be aggravated in many cases because such tractors typically are equipped with rear tires of relatively large diameter and/or ribbed or lugged treads capable of exerting substantial traction, especially on relatively soft unpaved surfaces. Such tractors frequently have sufficient power and traction to climb hills steeper than the angle at which the tractor will overturn.
Prior art attempts to overcome the foregoing problem generally have centered around preventing its occurrance, and have been less than satisfactory in many applications. The usual prior-art approach is to move the center of gravity of the tractor further in front of the back axle, thereby increasing the maximum angle of attack before overturning becomes theoretically possible. While this increased maximum angle of attack delays the onset of overturning, the increased maximum angle of attack is such that the front end of the tractor already is well above the normal level position and the tractor operator is leaning back at an unnatural attitude when the tractor reaches this maximum angle. The overturning or tipping movement at this point can be sudden and drastic, frightening the tractor driver and throwing him further off balance, and preventing any possible corrective action which the driver might take to prevent overturning when an incipient maximum angle of attack is reached.
Other approaches of the prior art have included mounting roll bars or cages about the operator's position on the tractor, in an attempt to protect the operator if overturning takes place. While these latter measures are commendable, they fail to address the basic problem, namely, how to avoid the onset of overturning in the first place.